Zoonotic tapeworms

Taenid tapeworms are helminth endoparasites belonging to order Platyhelminthes (flatworms).

The Taenids are unique in having mammals as both definitive (harboring adult parasites) and
intermediate (harboring larval parasites) hosts.
Their transmission and life cycles are linked to specific predator–prey
assemblages: a carnivorous or omnivorous definitive host
gets infected by eating its prey, an intermediate host.
Switching among carnivorous definitive hosts
(e.g., among canids, felids, hyaenids and humans)
that have historically exploited common prey resources played
a crucial role in Taenid speciation
(formation of new biological species).

To answer the question how species of human-specific taenids arose,
it was proposed initially that in early
times of animal husbandry (not more than 10,000 years ago),
tapeworms circulating between dogs and
domesticated ruminants were the source of first Taenia that
colonized humans.
More recent phylogenetic studies based on molecular data had
estimated age for divergence of human-specific T. saginata and T. asiatica
at 0.78–1.71 million years ago which suggest
that the parasites adapted to Homo sapiens
much earlier than domestication started.
Shift of human diet from herbivory to omnivory and, later,
to facultative carnivory 2.0–2.5 million ago was necessary for the adaptation
of the tapeworms to human host.

Maintenance of T. solium within populations of Homo may have further
been facilitated by cannibalism in a unique "human–human cycle".

T. asiatica as a separate species was discovered relatively recently.
It had been mistaken for Taenia saginata Goeze (1782) for more than 200 years and later was classified
as subspecies of T. saginata. However,
extensive epidemiological, molecular, comparative morphology and phylogenetic analyses
have indicated the independent and specific status of T. asiatica (Eom & Rim 1993).

Species of Taenia were among the earliest recognized helminths in humans, with written records
of their occurrence extending into antiquity.

Many tapeworms have indirect life cycle that requires an intermediate host for
larval stage of the parasite.

Usually the larvae of the tapeworm develop in flesh of prey animals (intermediate hosts), where
asexual reproduction of the parasite can occur, and sexual egg-laying adult
develops in intestinal tract of the predator.

There are three species of taenids that use humans as definitive hosts: Taenia solium,
T. saginata, an T. asiatica.
In addition, Taenia solium can also use humans as intermediate hosts causing cysticercosis.
This ability
makes this parasite especially important.
Other species of taenids circulate in wild animals and infect humans only occasionally causing
cysticercoses/coenuroses as well as taenoses.

Life cycles of all taenids are very similar. T. solium is distinguished by
its ability to use humans not only as definitive hosts but also as intermediate hosts.
Because of this, the species is most important within this group of parasites.
There is currently no evidence that cysticerci of either T. saginata or T.
asiatica can develop in humans.

egg
The eggs and distal gravid proglottids (section of worm's body
full of infective eggs) are released in feces and can
contaminate hands, water and food. Each egg contains an
an oncosphere (embryo). Egg size: 30 - 40 µm.
Each egg has hooklets and a striated outer shell.
The eggs of different species are very difficult to differentiate.
Eggs that are highly resistant to
ambient environmental conditions, remaining viable for up to 8 months in warm and humid climates.

larva (metacestode)
The oncospheres released from eggs by the action of stomach trypsin
cross the bowel wall, enter the bloodstream, and are carried to the tissues.
At small terminal vessels, they establish and encyst as cysticerci,
reaching their final size (0.5-2 cm) in 2–3 months.
Cysticercus is a tapeworm larva that consists of
fluid-filled vesicle, which contains single protoscolex as opposed to
coenurus - a vesicle that contains multiple inverted protoscolices, attached to the
internal membrane of the cyst. Daughter cysts may be seen inside some coenuri.
Coenuri are characteristic to some zoonotic tapeworms (see below).

Porcine cysticercosis (T. solium)
Cysticerci of T. solium establish themselves
in the muscles (including cardiac and tongue) and central nervous system (spinal cord and brain).
Most pigs are killed before the age of 9 months, which is too short period for
detectable illness to develop even in case of massive infection.
Commonly practiced way to diagnose the infection in pigs is
tongue examination: visual inspection and palpation for cysts is easy and accurate when
performed by a trained individual. Meat inspection and immunodiagnosis
are also very useful ante-mortem strategies.

Human cysticercosis
(T. solium)
Human cytocercosis has same mechanism as the porcine cytocercosis outlined above except that
infected people usually live long enough to develop symptoms.
Cysts are uniformly rounded or oval vesicles, varying in size from a few millimetres to 1–3 cm
(in rare cases a growing cyst reaches several centimetres in diameter).
Vesicles vary in contents according to their evolutionary stage.
Viable cysts have a translucent membrane through which the scolex is visible as a small 2–3 mm nodule.
When the cyst starts to degenerate, the vesicular fluid becomes
opaque and dense, and the edges of the cyst become irregular and shrink.
After calcification is complete, the cyst becomes round, whitish nodule.
Cysticercosis become symptomatic only when cysts are dying or become large enough to interfere with
vital organs. The incubation period for cysticercosis can last from 10 days to 10 years.

Neurocysticercosis (NCC)

NCC is currently considered the most common parasitic disease of the central nervous
system especially in endemic regions of Latin America, India, Asia,
Eastern Europe and Africa.
In NCC the cysts can be located about anywhere in the central nervous system (brain and spinal cord).
Epileptic seizures are the most common presentation of neurocysticercosis and generally represent the primary
or sole manifestation of the disease. Seizures occur in 50–80% of patients with parenchymal brain cysts or
calcifications.
Occasionally, a cyst grows larger than the usual 1–2 cm and acts in the same way as a tumoral mass (giant cyst).
The racemose form is a large translucent vesicle, frequently lobulated, without a scolex, which develops
in the basis of the brain or in the ventricles.
The racemose form is associated with high mortality.

Extraneural cysticercosis

Outside the central nervous system, cysticercosis causes no major symptoms. Subcutaneous cysticercosis
presents
as small, movable, painless nodules that are most commonly noticed in the arms or chest.
After a few months or even years,
the nodules become swollen, tender, and inflamed, and then they gradually disappear.

Ophthalmic cysticercosis

T. solium is the most common intraorbital parasite.
Intraocular cysts are most frequently found floating freely in the eye volume.
Visual disturbances and even blindness are related to the degree of damage to retinal tissue or
the development of chronic uveitis.

Bovine cysticercosis
(T. saginata)
Bovine cysticercosis is the source of substantial production losses,
condemnation and destruction of infected cattle.
Substantial economic impacts stem from increasing
labor costs of handling infected carcasses and processes that are required to render
cysticerci non-viable.

Porcine cysticercosis
(T. asiatica)
Oncospheres of T. asiatica exhibit viscero-tropism rather than
musculo-tropism. Metacestodes reside in
visceral organs such as the liver, spleen, omentum, and lungs, but not in the muscles.

adult
Association between cysticercosis and taenosis, which separately were known for ages,
was found only in 1855 by Kuchenmaister.
He fed condemned prisoners with cysticercosis-infected pork and recovered young tapeworms at autopsy.

Transmission

Taeniasis occurs after consumption of undercooked animal product
infected with cysticerci: meat in case of T. solium and T. saginata and
liver or other internal organs in case of T. asiatica.

Growth and proliferation

Soon after ingestion larvae reach small intestine where they evaginate and release the head (scolex),
which attaches to the mucosa.
From proliferating cells in the neck the scolex begins forming segments (proglottids) in the
process that is known as strobilation.
Hermaphroditic proglottids undergo continuous differentiation to eventually form gravid
proglottids full of infective eggs.
Tapeworms take up most of their nutrients from
the host intestinal submucosa and detachment of the scolex from the
site causes death of the tapeworm and its elimination. Attached intact scolex stripped of all its
proglodits can regenerate.
The outstanding feature of tapeworms is the syncytial
nature of their tissue (cells are not completely separated exhibiting a cytoplasmic continuity between them).
One of the many controversial issues about human tapeworm
physiology is how the distal end of the strobila, which may be several meters away from the scolex,
acquires nutrients. The distribution of cytoplasmic glycogen in experimental strobilae, suggests
that the young proglottids may have abundant stores
of glycogen, which become depleted as they are maturing. Adult tapeworms can grow
up to 20 meters long, usually they reach 2-7 meters.

Duration and symptomps

Infections in humans may be of long duration, perhaps exceeding 20–25 years, in the
absence of treatment.
Clinical signs are limited, but
may include nausea, dizziness, altered appetite, headache and other manifestations.
Often infection is revealed by the spontaneous release of gravid proglottids.
Toxic effects from parasite metabolites have also been reported.
Most, carriers of T. solium will neither look for medical
care nor notice the tapeworm segments in their stools. Conversely, most patients infected with
T. saginata notice passage of proglottids, which are motile, more numerous, and larger than those
of T. solium.

application of untreated human sewage sludge onto pastures and other areas where animals are allowed to roam

indirect contamination of food products or water for cattle

dissemination of parasite eggs by birds from sewage sources to feed bins, silage and cattle pens

Human hosts may release
several segments daily, each containing thousands of eggs, and a single infected agricultural
worker can be responsible for epizootic outbreaks in cattle or swine.
Notably, eggs may remain infective in the environment for several months under suitable
conditions.

Although all tapeworms represent a considerable animal and human health burden,
T. solium remains as a primary target for eradication world-wide because of its ability to cause
cysticercosis in humans. Autoinfection (when individual parasitized by adult worm ingests
eggs expelled from its own body and becomes ill from cysticercosis) is considered a common occurence.
The following characteristics of human-specific tapeworms make them vulnerable to eradication:

the life cycle requires humans as its definitive host

tapeworm infection in humans is the only source of infection for pigs and cattle,
the natural intermediate hosts

domestic pigs and cattle, the intermediate hosts, can be managed

no significant wildlife reservoir exists

practical intervention is available in the form of chemotherapy
for human taeniosis and porcine cysticercosis with safe and effective drugs

Human cysticercosis

Because extraneural cyctocercosis is usually benign, in majority of cases no treatment is necessary.
However, because a proportion of people with cysticercosis may also be T. solium tapeworm carriers it may
be unavoidable.
Neurocysticercosis is associated with substantial morbidity and mortality and often requires treatment in
order to alleviate symptoms and improve quality of life of infected people.
The management of NCC may involve:

Antiparasitic drugs. Antiparasitic treatment may be unnecessary or even
contraindicated in some NCC patients (for example, in case of ocular or spinal cysts)
while in others, it may be beneficial and even lifesaving.
Between available antiparasitic drugs, niclosamide and praziquantel, niclosamide is the drug
of choice because it is not absorbed from the intestines.
With praziquantel there is a small risk that asymptomatic viable brain cysts
are affected by the drug in serum, causing neurological symptoms such as headache and seizures.

Steroids are usually administered in conjunction with antiparasitic therapy
to suppress inflammation caused by disintegrating cysts.

Antiepileptic drugs are the principal therapy for seizures associated with NCC, which may eventually be
lessened or even dissapear with time. The presence of calcifications in the brain is a known risk factor for
seizure relapse and the proportion of calcifying cysts may be higher in cysticercosis patients treated with
antiparasitics.

Surgery is performed when cysts can be safely removed. Surgery is usually
required for treating hydrocephalus caused by NCC and is also appropriate for removal of ventricular,
spinal or ophthalmic cysticerci.

Human taenasis

Both niclosamide (2 g orally in a single dose) and praziquantel (5 mg/kg orally in a single dose)
are effective against the adult tapeworm. Praziquantel may reach serum levels high enough
to affect brain cysts if the tapeworm carrier also has NCC, causing seizures or severe headaches.
An elctrolyte-polyethylenglycol salt (EPS) purge is usually indicated short after the chemotherapy
to expel the worm and avoid the potential risk of having proglottids or eggs returned to
the stomach and digested, leading to internal autoinfection. It also
greatly improves the recovery of tapeworm scolices and gravid proglottids which indicates cure as well as
provides parasite material for species identification of the infecting worm.

Porcine cysticercosis

A veterinary benzimidazole and oxfendazole, is more than 95% effective in killing the cysts
in the pig when given in a single dose of 30 mg/kg.
Infested meat in oxfendazole-treated pigs needs at least eight weeks for all the cysts to
degenerate and up to 12 weeks to achieve a clear, acceptable appearance of the pork for human consumption.

Bovine cysticercosis

Oxfendazole was developed for use as an anthelmintic for ruminants and is
administered to cattle at an oral dose of 4.5 mg/kg.